Much of today's high-speed data communication traffic is carried out in two directions simultaneously; that is, the traffic is full duplex. For data speeds below about 2400 bits per second the voiceband extending from approximately 300 to 3000 Hz can be divided in half to allow dedication of each half to a particular transmission direction on an equivalent four-wire transmission line. For data speeds above 2400 bits per second two physically separate two-wire lines of voice bandwidth are required. If two-way (full duplex) high-speed simultaneous communication between two points could be accomplished at full bandwidth on a single two-wire channel, significant cost savings would become possible. Furthermore, a capability for simultaneous two-way digital data communication on the public switched direct-distance dialing (DDD) network would be valuable in cases where the data customer could benefit from zero "turn-around" time. Under present practices reversing the direction of transmission on a half-duplex DDD transmission channel requires time to disable built-in echo suppressors.
Two-way data communication on a two-wire transmission channel requires suppression of the interfering locally generated signal at the receiver input of each data terminal. This is partly accomplished by the use of hybrid coupling networks at the terminals, but a residual interference results from the inevitable impedance mismatch between a fixed hybrid coupler and a variety of channel connections, and from echoes returning from distant points in the transmission channel.
Adaptive echo cancellers implemented by transversal filters have been proposed for analog facilities by, for example, J. L. Kelly, Jr., and B. F. Logan, Jr., in U.S. Pat. No. 3,500,000 issued Mar. 10, 1970. In this echo canceller a portion of the analog signal incoming to a hybrid coupling on the four-wire side is passed through a transversal filter with adjustable tap gain control to synthesize a cancellation signal for subtraction from the signal outgoing from the hybrid coupling. The resultant outgoing signal is clipped and correlated with the sequence of samples of the incoming signal appearing at the taps of the transversal filter to form control signals for the tap gains or weighting coefficients of the transversal filter.
A similar arrangement is described by F. K. Becker and H. R. Rudin in the Bell System Technical Journal (Vol. 45, 1966, pp. 1847-1850) in a paper entitled "Application of Automatic Transversal Filters to the Problem of Echo Suppression." Results achieved with a practical realization of the transversal filter as an adaptive echo canceller are reported by V. G. Koll and S. B. Weinstein in Institute of Electrical & Electronics Engineers Transactions on Communications, (Vol. COM-21, No. 2, 1973, pp. 143-147) in a paper entitled "Simultaneous Two-Way Data Transmission Over a Two-Wire Circuit."
The above-mentioned echo cancellers employing transversal structures require tap spacings no larger than the Nyquist interval which is equal to the reciprocal of twice the highest frequency present in the message signal to be transmitted, principally because the signal to be transmitted is analog in nature. In the typical voice bandwidth of 4000 Hz, eight such taps are required for each millisecond of anticipated echo delay.
In the copending U.S. patent application of K. H. Mueller (Ser. No. 636,297, filed Nov. 28, 1975), now U.S. Pat. No. 4,087,654, an echo cancelling signal is synthesized in a transversal structure having taps spaced at baud intervals, rather than at the Nyquist intervals specified in the prior art, from samples of the baseband data to be transmitted prior to modulation or application to the hybrid coupling under the control of an error signal obtained at the output of the receiver portion of a data transceiver.
In the application of D. D. Falconer and S. B. Weinstein Ser. No. 720,999, and now U.S. Pat. No. 4,074,086, filed Feb. 14, 1978 filed concurrently with the parent application Ser. No. 721,032 a joint echo canceller and intersymbol interference equalizer are incorporated into the receiver section of a data terminal, such that the intersymbol interference equalizer precedes the echo canceller and the adaptation of both is jointly controlled by the detected baseband output of the receiver section. By this arrangement the cancellation signal is effective only at baud or symbol intervals, as is the companion intersymbol interference equalizer, with the result that the near-end and far-end timing signals must be closely synchronized. Otherwise, elastic buffering between the cancellation signal, derived from the near-end transmitted data timing train, and the equalized received signal, whose timing is determined at the far-end terminal or by intermediate signal repeaters, must be provided. A suggested arrangement for such buffering is described in the Falconer-Weinstein application.
It is an object of this invention to provide an echo or leakage canceller in a two-way, two-wire digital data transmission system which is independent of far-end transmitter timing waves.
It is another object of this invention to provide an echo canceller in a two-way, two-wire data transmission system which suppresses echoes over the entire signal bandwidth and not merely at baud intervals.
It is a further object of this invention to provide echo cancellation in a two-way, two-wire data transmission system independently of parameters of the receiver proper.
It is yet another object of this invention to accommodate widely separated near-end and far-end echo components by inclusion of a passive bulk delay section between active echo-cancellation sections for each of such separated components.